The present disclosure relates to geophone devices for sensing vibrations in earth formations, and may be applicable to other types of vibration transducers, either in sensing or transmitting operation. More specifically, the present disclosure relates to a damping controlled geophone.
In seismic exploration, vibrations in the earth resulting from a source of seismic energy may be sensed at discrete locations by sensors and the output of the sensors used to image underground structures, or to locate seismic events. The source of seismic energy can be natural, such as earthquakes and other tectonic activity, subsidence, volcanic activity or the like, or man-made such as acoustic noise from surface or underground operations, or from deliberate operation of seismic sources at the surface or underground. Seismic sensors fall into two categories: hydrophones that sense the pressure field resulting from a seismic source, or geophones that sense vibration arising from a seismic source.
An oscillatory geophone is shown in FIG. 1-1. The geophone 10 includes a housing 24, a top cap 27 and a bottom cap 28 that are affixed to the housing 24, a magnet 15 having a pair of pole pieces 16, 18 that are respectively affixed to the top and bottom caps 27, 28, a moving coil that includes two coil windings 12, 13 and a bobbin 14 with suspension springs 20, 22 as shown in FIG. 1-1. The magnet 15, along with its two pole pieces 16, 18, can move with the housing 24. Pole pieces 16, 18 and housing 24 are made of magnetically permeable material and form a magnetic flux 25 in which the moving coil is suspended. The caps 27, 28 are made of magnetically impermeable material. In this particular embodiment as shown, the coil windings 12, 13 are connected in series to form a continuous coil. The windings 12, 13 may be disposed about the bobbin 14 in opposite directions, so that the windings 12, 13 can generate voltage in a common direction. The windings 12, 13 of the moving coil are commonly mounted and move together. As illustrated, the magnetic flux 25 passes out one winding from inside to outside near the north of the magnet 15, and then out the other winding from outside to inside near the south of the magnet 15.
When the earth moves due to the seismic energy propagating either directly from the source or via an underground reflector, the geophone 10, which can be located at the earth's surface or on the wall of a borehole penetrating the earth, moves in the direction of the particle motion resulting from propagation of the energy. If the axis of the geophone 10 is aligned with the direction of motion, however, the coil windings 12, 13 mounted on the springs 20, 22 inside the geophone 10 stay in the same position causing relative motion of the moving coil with respect to the magnetic flux 25 that moves with the housing 24. When the moving coil moves in the magnetic field, a measurable voltage is induced in the moving coil, which is proportional to the velocity of the relative motion between the moving coil and the magnetic flux 25.
Variations of geophones are described in U.S. Pat. No. 7,099,235 to Kamata, U.S. Publication 2011/0007608 to Woo, and U.S. Pat. No. 4,159,464 to Hall.